Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
Prior Art FIG. 1 shows a typical spin valve sensor 100 (not drawn to scale) comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) 125. An underlayer 126 is positioned below the AFM layer 125.
The underlayer 126, or seed layer, is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate. A variety of oxide and/or metal materials have been employed to construct underlayer 126 for improving the properties of spin valve sensors. Often, the underlayer 126 may be formed of tantalum (Ta), zirconium (Zr), hafnium (Hf), or yttrium (Y). Ideally, such layer comprises NiFeCr in order to further improve operational characteristics.
Free layer 110, spacer 115, pinned layer 120, the AFM layer 125, and the underlayer 126 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensor 170 is connected to leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the spin valve effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
Prior Art FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Flanking each side of the electrodes (202, 204) are hard bias layers 282 which serve to promote stability of the MJT sensor 200 as well as provide a strong coupling between the ferromagnetic layers thereof. Typically, the hard bias layers 282 are traditionally constructed using a material that is at least partially conductive. Thus, an undesirable shunt current, Ish, flows between the first and second leads 260 and 265.
Prior Art FIG. 2A shows the prior art MJT sensor 200 of FIG. 2 with insulating layers 290 to prevent the shunt current. As shown, the electrodes (202, 204) are coupled to the first and second leads 260 and 265, as set forth in FIG. 2. However, the insulating layers 290 provide an insulating buffer between the hard bias layers 282 and the first and second leads 260 and 265 to prevent the shunt current.
In use, the aforementioned hard bias layers 282 promote stability of the MJT sensor 200 by way of a hard bias magnetic field 295 (see FIG. 2A) along a track width 296 of the sensor 200. The strength of such hard bias magnetic field 295 is a function of a relationship between the thicknesses of the free layer 210 and hard bias layers 282 (see FIG. 2).
Prior Art FIG. 2B illustrates a graph showing the strength of a hard bias magnetic field 295 over the length of the track width 296 of the sensor 200, for two thickness relationships (i.e. where the thickness of the hard bias layers 282 is 5 and 10 times that of the free layer 210). As shown, in both cases, the hard bias magnetic field 295 is greater adjacent ends of the track width 296 to promote the aforementioned stability. Moreover, the hard bias magnetic field 295 is less adjacent a center of the track width 296 to promote a sensitivity of the sensor 200.
There has been an on-going trend of manufacturing smaller sensors 200 with inherently shortly track widths 296. To ensure that the aforementioned stability is preserved (i.e. by maintaining the strength of the hard bias magnetic field 295 at the ends of the track width 296), the thickness of the hard bias layers 282 has not been reduced in concert with the foregoing trend of smaller sensors 200. Unfortunately, this has caused the strength of the hard bias magnetic field 295 to also be maintained at the center of the track width 296, resulting in reduced sensitivity. See, for example, the difference between the two cases shown in FIG. 2B (i.e. where the thickness of the hard bias layers 282 is 5 and 10 times that of the free layer 210).
There is thus a need for continuing with the aforementioned trend of smaller sensors and respectively large hard bias layers (to maintain stability), while also maintaining the sensitivity of the sensor.